Committed ventricular safety pacing

ABSTRACT

A method and apparatus provides a sensed AV delay and/or a paced AV delay following an atrial sensed event and/or an atrial paced event, respectively. The AV delay is a predetermined time period initiated by the respective occurrence of the atrial sensed event or atrial paced event. A ventricular safety pacing window is defined during at least an initial portion of the AV delay. Ventricular events are sensed during the AV delay. If a ventricular event is sensed during the ventricular safety pacing window, then a commitment is made to the delivery of a ventricular safety pace upon expiration of the AV delay.

FIELD OF THE INVENTION

[0001] The present invention relates generally to implantable medicaldevices and methods of cardiac stimulus. More particularly, the presentinvention pertains to implantable medical pacing devices and methodsthat employ ventricular safety pacing (VSP) in cardiac stimulation.

BACKGROUND OF THE INVENTION

[0002] Generally, in the human heart, the sinus (or sinoatrial (SA) nodetypically located near the junction of the superior vena cava and theright atrium) constitutes the primary natural pacemaker by whichrhythmic electrical excitation is developed. The cardiac impulse arisingfrom the sinus node is transmitted to the two atrial chambers (or atria)at the right and left sides of the heart. In response to excitation fromthe SA node, the atria contract, pumping blood from those chambers intothe respective ventricular chambers (or ventricles). The impulse istransmitted to the ventricles through the atrioventricular (AV) node,and via a conduction system comprising the bundle of His, or commonbundle, the right and left bundle branches, and the Purkinje fibers. Thetransmitted impulse causes the ventricles to contract with the rightventricle pumping unoxygenated blood through the pulmonary artery to thelungs, and the left ventricle pumping unoxygenated (arterial) bloodthrough the aorta and the lesser arteries to the body. The right atriumreceives the oxygenated (venous) blood. The blood oxygenated by thelungs is carried via the pulmonary veins to the left atrium.

[0003] The above action is repeated in a rhythmic cardiac cycle in whichthe atrial and ventricular chambers alternately contract and pump, andthen relax and fill. One-way valves, between the atrial and ventricularchambers on the right and left sides of the heart, and at the exits ofthe right and left ventricles, prevent backflow of the blood as it movesthrough the heart and the circulatory system. This sinus node isspontaneously rhythmic, and the cardiac rhythm it generates is termedsinus rhythm. This capacity to produce spontaneous cardiac impulses iscalled rhythmicity. Some other cardiac tissue possess rhythmicity andhence constitute secondary natural pacemakers, but the sinus node is theprimary natural pacemaker because it spontaneously generates electricalpulses at a faster rate. The secondary pacemakers tend to be inhibitedby the more rapid rate at which impulses are generated by the sinusnode.

[0004] Disruption of the natural pacemaking and propagation system as aresult of aging or disease is commonly treated by artificial cardiacpacing, by which rhythmic electrical discharges are applied to the heartat a desired rate from an artificial pacemaker. A pacemaker is a medicaldevice which delivers electrical pulses to an electrode that isimplanted adjacent to or in the patient's heart to stimulate the heartso that it will contract and beat at a desired rate. If the body'snatural pacemaker performs correctly, blood is oxygenated in the lungsand efficiently pumped by the heart to the body's oxygen-demandingtissues. However, when the body's natural pacemaker malfunctions, animplantable pacemaker often is required to properly stimulate the heart.

[0005] Implantable pacemakers are typically designed to operate usingvarious different response methodologies, such as, for example,nonsynchronous or asynchronous (fixed rate), inhibited (stimulusgenerated in the absence of a specified cardiac activity), or triggered(stimulus delivered in response to a specific hemodynamic parameter).Generally, inhibited and triggered pacemakers may be grouped as“demand”-type pacemakers, in which a pacing pulse is only generated whendemanded by the heart. To determine when pacing is required by thepacemaker, demand pacemakers may sense various conditions such as heartrate, physical exertion, temperature, and the like. Moreover, pacemakerimplementations range from the simple fixed rate, single chamber devicethat provides pacing with no sensing function, to highly complex modelsthat provide fully-automatic dual chamber pacing and sensing functions.For example, such multiple chamber pacemakers are described in U.S. Pat.No. 4,928,688 to Mower entitled “Method and Apparatus for TreatingHemodynamic Dysfunction,” issued May 29, 1990; U.S. Pat. No. 5,792,203to Schroeppel entitled “Universal Programmable Cardiac StimulationDevice,” issued Aug. 11, 1998; U.S. Pat. No. 5,893,882 to Peterson etal. entitled “Method and Apparatus for Diagnosis and Treatment ofArrhythmias,” issued Apr. 13, 1999; and U.S. Pat. No. 6,081,748 toStruble et al. entitled “Multiple Channel, Sequential Cardiac PacingSystems,” issued Jun. 27, 2000.

[0006] For example, a DDD pacer paces either chamber (atrium orventricle) and senses in either chamber. Thus, a pacer in DDD mode, maypace the ventricle in response to electrical activity sensed in theatrium. Further, for example, a pacer operating in VVI mode, paces andsenses in the ventricle, but its pacing is inhibited by spontaneous andelectrical activity of the ventricle (i.e., intrinsic ventricularactivity or events, wherein the ventricle paces itself naturally).

[0007] As such, it may be desired to sense in one cardiac chamber (e.g.,detect electrical activity represented of contraction of a chamber andreferred to as a “sensed event”) and, in response, pace (referred to asa “paced event”) in the same or different chamber. It also may bedesired to pace at two electrode locations following a sensed event atone of the pacing electrodes or at a different electrode. For example,patients are often treated with pacemakers that include an electrode ineach of the two atrial chambers and a third electrode in the rightventricle. Both atrial chambers usually are paced following a sensedevent in either chamber.

[0008] Further, bi-ventricular pacing devices are also used fortreatment of patients. For example, in such a bi-ventricular pacingapparatus, multiple implantable leads having electrodes associated witha part thereof are implanted to the respective chambers of a patient'sheart and coupled to respective circuitry for forming multiple channelsfor pacing and sensing, e.g., left ventricular channel, right atrialchannel, etc. Such an exemplary implantable, four-channel cardiacpacemaker is described in U.S. Pat. No. 6,070,101 to Struble et al.entitled “Multiple Channel, Sequential Cardiac Pacing Systems,” issuedMay 30, 2000. For example, the distal end of a right atrial lead isattached to the right atrial wall and a right ventricular lead is passedthrough a vein into the right atrial chamber of the heart and into theright ventricle where its distal electrodes are fixed. Another lead ispassed through a vein into the right atrial chamber of the heart, intothe coronary sinus (CS), and then inferiorly into the great vein toextend a distal pair of left ventricular pace/sense electrodes alongsidethe left ventricular chamber and leave a proximal pair of left atrialpace/sense electrodes adjacent the left atrium. With such electrodeplacement, pacing and sensing can be performed in each chamber of theheart, enabling bi-ventricular pacing. For example, such bi-ventricularpacing may be performed following atrial sensed events or atrial pacedevents.

[0009] Typically in such types of pacing apparatus, if an intrinsic orpacing pulse occurs in one of the chambers, for example, the atrium,then this activity may be erroneously sensed in the other chambers dueto cross-talk. In order to eliminate this type of error, in the past,pacemakers have been provided with blanking periods for blanking thesensor in one channel after a pacing pulse occurs in the other. Thisblanking period is usually referred to as the cross-channel blankingperiod. Following the blanking period, an alert period is normallydesignated during which the cardiac chamber of interest is monitored forintrinsic activity. If no such activity is sensed by the end of thisalert, then a pacing pulse is applied to the chamber. However, oneproblem with such pacemakers and the use of blanking channels has beenselecting the duration of the blanking period for a particular channelproperly. If the blanking period is too short, a cross-channel artifactcould be interpreted as an intrinsic activity and therefore pacing maybe erroneously inhibited. On the other hand, if the blanking period istoo long, intrinsic activity may be missed and the chamber may be pacedwhen no such pacing is required. Either situation is undesirablephysiologically.

[0010] Yet further, particularly in bi-ventricular pacing systems, e.g.,systems which provide delivery of ventricular stimulus to bothventricular chambers following paced or sensed atrial events, a leftventricle lead is typically placed as described above, in a cardiac veinvia the coronary sinus. Since the lead tip is in close proximity to thecoronary sinus tractus, far-field coronary sinus/left atrial signals ofsignificant amplitudes can potentially be sensed as ventricular activityand present inappropriate inhibition of bi-ventricular pacing. Forexample, in particular, when bipolar left ventricle leads are employed,the anode ring of the bipolar lead can be close to/or just within thecoronary sinus system depending on tip-ring distance for the electrodeson the left ventricle lead. With the leads positioned in such a manner,atrial activity may be sensed using the left ventricle electrodes, takenas an intrinsic left ventricle event, and prevent or inhibit delivery ofventricular stimulus.

[0011] Further, for example, lead dislodgment may also lead to suchmistaken sensing of ventricular events. For example, a left ventricularlead may be placed via the coronary sinus with a passive lead tip fixedin a cardiac vein. Leads are typically placed 1 to 4 centimeters withinthe vessels (or, generally, as far as possible). Either partial leaddislodgment (e.g., gradual pullback) or permanent lead dislodgment mayresult in an electrode location that is undesirable and conducive toover-sensing of left atrial activity. Therefore, once again, suchover-sensing of atrial activity may lead to falsely sensed ventricularactivity and the inhibition of the delivery of ventricular stimulus. Assuch, bi-ventricular stimulation may be intermittently or may becompletely lost.

[0012] In many pacing apparatus, such as, for example, dual chamberpacing devices operating in DDD mode, ventricular safety pacing (VSP) isgenerally available and intended to prevent inappropriate inhibition ofventricular pacing by ensuring that an atrial paced event is followed bya ventricular paced event. When this VSP feature is on, ventricularsensing within a VSP window of typically 110 milliseconds following anatrial paced event causes ventricular pacing at the end of the VSPwindow (e.g., the 110 millisecond period).

[0013] For example, if the pacing apparatus is programmed with a pacedAV interval (PAV=100 milliseconds) (i.e., the AV interval following anatrial paced event and defined as the time between the paced event anddelivery of ventricular stimulus) that is less than the VSP window(e.g., 110 milliseconds), then the delivery of ventricular stimuluswould occur at the end of the programmed PAV interval when ventricularsensing occurs during the VSP window.

[0014] In another example, if the pacing apparatus is programmed with aPAV interval (PAV=150 milliseconds) that is greater than the VSP window(VSP=110 milliseconds), then when ventricular sensing occurs during theVSP window, the delivery of ventricular stimulus would occur at the endof the VSP interval, and not at the time out of the PAV interval.

[0015] Further, with such conventional VSP, if no ventricular events aresensed during the VSP window and the PAV is greater in length than theVSP window, if a ventricular event is sensed during the PAV but afterthe VSP window, delivery of ventricular stimulus would be inhibited dueto the sensed intrinsic ventricular event. This VSP feature is designedto ensure ventricular output in the event of noise sensing on theventricular lead (e.g., cross-talk) within the VSP window or 110milliseconds after an atrial paced event and outside the programmedventricular blanking period.

[0016] Another example of a pacemaker having safety pacing is describedin U.S. Pat. No. 5,782,881 to Lu et al., issued Jul. 21, 1998 andentitled “Pacemaker With Safety Pacing.” As described therein, amonitoring window is defined during an AV delay during which signalssensed in a ventricular channel are monitored. If an abnormal signal issensed during this window, certain features of the signal are analyzedto determine if its origin is intrinsic or due to cross-channel activityor noise. Cross-channel activity is ignored. If intrinsic cardiacactivity is identified, then no pacing pulse or ventricular stimulationis applied. If no decision can be made as to the source of the cardiacactivity, then delivery of stimulus is performed and ventricular pacingis not inhibited by the sensed activity.

[0017] The above-described VSP features may be inadequate in manycircumstances. For example, conventional VSP features typically onlyoccur when programmed on, and only following a paced atrial event. Inother words, a VSP window is only utilized following delivery of pacingstimulus in the atrium and thus during a PAV interval. Such VSP featuresdo not occur following atrial sensing or an atrial sensed event, where atimed sensed AV interval (SAV) is initiated.

[0018] Further, for example, in standard DDD pacemakers, when VSP isused, delivery of ventricular stimulus normally occurs following theexpiration of the VSP window of 110 milliseconds, or at the terminationof a PAV interval, whichever is less as previously described above. Ifthe optimum PAV is programmed greater than 110 milliseconds (i.e., thePAV interval is longer than the VSP window), then normal delivery ofstimulus at the termination of the 110 millisecond VSP window occurs toosoon and not at termination of an optimum programmed PAV. In otherwords, such stimulus delivery at the termination of the VSP window willforeclose control of the heart by any intrinsic activity that may occurbetween the end of the VSP window and the termination of the PAVinterval. As such, the optimized and programmed AV timing may be lostand competition to ventricular filling may occur.

[0019] Generally, optimized and programmed PAV intervals aresignificantly longer (e.g., 30-50 milliseconds) than SAV intervals.Fifty (50) milliseconds is generally required for slower conductionpatients, for example, heart failure (HF) patients with conductiondelays. For example, anticipated ranges for optimized SAV/PAV intervalsin many patients may be, for example, SAV 80-130 milliseconds/PAV130-180 milliseconds. Therefore, the problems associated with an optimumPAV programmed at a length greater than 110 milliseconds are readilyapparent.

[0020] Yet further, as VSP does not typically occur following atrialsensing as described above, then any inappropriate over-sensing ofintrinsic coronary sinus/left atrial signals during an SAV interval willresult in the mistaken belief that a ventricular event has occurred andventricular stimulation therapy is inhibited. This is particularlyundesirable in atrial bi-ventricular pacing which is typically intendedfor heart failure patients with left bundle branch block (LBBB) and/orintraventricular conduction delay (IVCD) and intact sinus rhythm (SR).The expected majority of the therapy to be delivered in such patients isgenerally associated with sensed atrial events and operating with an SAVinterval as opposed to paced atrial events operating with a PAVinterval. As therapy operation with a PAV following a paced atrial eventoccurs only in the minority of such patients, use of standard orconventional VSP features for atrial bi-ventricular pacing that resultonly after atrial paced events operating with a PAV interval is limited.

[0021] Table I below lists U.S. patents relating to multiple chamberpacing devices and devices and methods having VSP features. TABLE I U.S.Pat. No. Inventor Issue Date 4,928,688 Mower May 29, 1990 4,932,406Berkovits Jun. 12, 1990 4,944,298 Sholder Jul. 31, 1990 5,144,949 OlsonSep. 8, 1992 5,292,340 Crosby et al. Mar. 8, 1994 5,318,594 Limousine etal. Jun. 7, 1994 5,782,881 Lu et al. Jul. 21, 1998 5,792,203 SchroeppelAug. 11, 1998 5,893,882 Peterson et al. Apr. 13, 1999 5,902,324 Thompsonet al. May 11, 1999 6,070,101 Struble et al. May 30, 2000 6,081,748Struble et al. Jun. 27, 2000

[0022] All references listed in Table I, and elsewhere herein, areincorporated by reference in their respective entireties. As those ofordinary skill in the art will appreciate readily upon reading theSummary of the Invention, Detailed Description of the Embodiments, andclaims set forth below, at least some of the devices and methodsdisclosed in the references of Table I and elsewhere herein may bemodified advantageously by using the teachings of the present invention.However, the listing of any such references in Table I, or elsewhereherein, is by no means an indication that such references are prior artto the present invention.

SUMMARY OF THE INVENTION

[0023] The present invention has certain objects. That is, variousembodiments of the present invention provide solutions to one or moreproblems existing in the prior art with respect to implantable medicaldevice pacing techniques and, in particular, current ventricular safetypacing (VSP) techniques. One of such problems is that the current pacingapparatus generally apply VSP following only paced atrial events.Further, for example, pacing at the termination of the VSP window usingcurrent techniques may also not be adequate, e.g., when the PAV intervalis longer than the VSP window. Yet further, for example, other problemsinvolve the general inhibition of bi-ventricular stimulation therapy dueto inappropriate over-sensing of intrinsic coronary sinus/lower leftatrial signals during AV intervals and inhibition of ventricularstimulus due at least in part to lead dislodgment and inappropriatesensing thereafter.

[0024] In comparison to known VSP techniques, various embodiments of thepresent invention may provide one or more of the following advantages.For example, VSP according to the present invention always ensurespacing therapy at the optimized programmed SAV and PAV intervals, i.e.,SAV and PAV delays. Further, such committed delivery of ventricularstimulation therapy at the optimized SAV and PAV intervals is ensuredeven with the occurrence of inappropriate coronary sinus/left atrialfar-field sensing, even with the occurrence of post-atrial pacedringing, even with the occurrence and sensing of coronary sinus/leftatrial intrinsic signals; etc. This ensured ventricular pacing therapyat the optimized programmed SAV and PAV intervals is provided by notonly establishing committed VSP following a paced atrial event, but alsofollowing a sensed atrial event.

[0025] Some embodiments of the method of the present invention includeone or more of the following features: providing a sensed AV delayfollowing an atrial sensed event with the sensed AV delay being apredetermined time period initiated thereby; defining a VSP windowduring at least an initial portion of a sensed AV delay with the sensedAV delay further including a remainder portion thereof subsequent to theinitial portion; sensing ventricular events during the sensed AV delay;delivering ventricular stimulus upon expiration of the sensed AV delayif no ventricular events are sensed during the VSP window defined duringthe initial portion of the sensed AV delay and the reminder portionthereof; inhibiting the delivery of ventricular stimulus upon expirationof a sensed AV delay if no ventricular events are sensed during the VSPwindow but a ventricular event is sensed during a remainder portion ofthe sensed AV delay; committing to the delivery of a ventricularstimulus upon expiration of a sensed AV delay if a ventricular event issensed during a VSP window; defining a VSP window during an initialportion of a sensed AV delay that is 110 milliseconds; and defining asensed AV delay that is equal to or greater than a VSP window.

[0026] Other embodiments of the method of the present invention includeone or more of the following features: providing a paced AV delayfollowing an atrial paced event with the paced AV delay being apredetermined time period initiated thereby; defining a VSP windowduring at least an initial portion of the paced AV delay with the pacedAV delay further comprising a remainder portion thereof subsequent tothe initial portion; sensing ventricular events during the paced AVdelay; delivering ventricular stimulus upon expiration of the paced AVdelay if no ventricular events are sensed during a VSP window definedduring an initial portion of a paced AV delay and the remainder portionthereof; inhibiting the delivery of ventricular stimulus upon expirationof the paced AV delay if a ventricular event is not sensed during theVSP window but a ventricular event is sensed during the remainderportion of the paced AV delay; committing to delivery of ventricularstimulus upon expiration of the paced AV delay if a ventricular event issensed during the VSP window; defining a paced AV delay and a sensed AVdelay that are both greater than a VSP window; defining a paced AV delaythat is greater than the sensed AV delay; delivering bi-ventricularstimulus; and providing an AV delay initiated by occurrence of either anatrial sensed event or an atrial paced event with the AV delay being aprogrammed time period whose length is varied depending upon whether theAV delay is initiated by the occurrence of an atrial sensed event or isinitiated by the occurrence of an atrial paced event.

[0027] Further, some embodiments of an apparatus according to thepresent invention include one or more of the following features: a dualchamber pacing apparatus; atrial pacing and sensing circuitry togenerate atrial pacing pulses and sense atrial events; ventricularpacing and sensing circuitry to generate ventricular pacing pulses andsense ventricular events; control circuitry operable to provide a sensedAV delay following an atrial sensed event detected using the atrialsense circuitry with the sensed AV delay being a predetermined timeperiod initiated by the atrial sensed event; control circuitry operableto define a VSP window during at least an initial portion of the sensedAV delay with the sensed AV delay further including a remainder portionthereof subsequent to the initial portion; control circuitry operable todetect ventricular events using the ventricular sensing circuitry duringa sensed AV delay; control circuitry operable to control delivery ofventricular stimulus using the ventricular pacing circuitry uponexpiration of a sensed AV delay such that if no ventricular events aredetected during the VSP window defined during the initial portion of thesensed AV delay and the remainder portion thereof then ventricularstimulus is deliver at the expiration of the sensed AV delay; controlcircuitry operable to control delivery of ventricular stimulus using theventricular pacing circuitry upon expiration of a sensed AV delay suchthat if a ventricular event is not detected during the VSP window but aventricular event is detected during the remainder portion of the sensedAV delay then delivery of ventricular stimulus upon expiration of thesensed AV delay is inhibited; and control circuitry operable to controldelivery of ventricular stimulus using the ventricular pacing circuitryupon expiration of a sensed AV delay such that if a ventricular event issensed by the ventricular sensing circuitry during the VSP window thenthe ventricular pacing circuitry is committed to delivery of ventricularstimulus upon expiration of the sensed AV delay.

[0028] Yet further, other embodiments of the apparatus may include oneor more of the following features: control circuitry operable to providea paced AV delay following an atrial paced event with the paced AV delaybeing a predetermined time period initiated by the occurrence of anatrial paced event resulting from the generation of atrial pacing pulsesby the atrial pacing circuitry; control circuitry operable to define aVSP window during at least an initial portion of a paced AV delay withthe paced AV delay further including a remainder portion thereofsubsequent to an initial portion; control circuitry operable to controldelivery of ventricular stimulus using the ventricular pacing circuitryupon expiration of a paced AV delay such that if no ventricular eventsare detected during the VSP window defined during the initial portion ofthe paced AV delay and the remainder portion thereof then ventricularstimulus is deliver at the expiration of the paced AV delay; controlcircuitry operable to control delivery of ventricular stimulus using theventricular pacing circuitry upon expiration of a paced AV delay suchthat if a ventricular event is not detected during the VSP window but aventricular event is detected during the remainder portion of the pacedAV delay then delivery of ventricular stimulus upon expiration of thepaced AV delay is inhibited; control circuitry operable to controldelivery of ventricular stimulus using the ventricular pacing circuitryupon expiration of a paced AV delay such that if a ventricular event issensed by the ventricular sensing circuitry during the VSP window thenthe ventricular pacing circuitry is committed to delivery of ventricularstimulus upon expiration of the paced AV delay; bi-ventricular pacingand sensing circuitry to generate bi-ventricular pacing pulses and senseventricular events; and control circuitry operable to provide an AVdelay initiated by occurrence of either an atrial sensed event detectedby atrial sensing circuitry or an atrial paced event associated with thegeneration of atrial pacing pulses using atrial pacing circuitry withthe AV delay being a programmed time period whose length is varieddepending upon whether the AV delay is initiated by the occurrence of anatrial sensed event or is initiated by the occurrence of an atrial pacedevent.

[0029] The above summary of the present invention is not intended todescribe each embodiment or every implementation of the presentinvention. Advantages, together with a more complete understanding ofthe invention, will become apparent and appreciated by referring to thefollowing detailed description and claims taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The present invention will be further described with reference tothe drawings, wherein:

[0031]FIG. 1 is an implantable medical device (IMD) in accordance withone embodiment of the present invention, wherein the IMD is implantedwithin a body of a patient;

[0032]FIG. 2 is an enlarged view of the IMD of FIG. 1 diagrammaticallyillustrating coupling with the patient's heart in accordance with oneembodiment of the invention;

[0033]FIG. 3 is a functional block diagram of an IMD in accordance withone embodiment of the present invention;

[0034]FIG. 4 is an IMD in accordance with another embodiment of theinvention, wherein the IMD is an implantablepacemaker/cardioverter/defibrillator (PCD);

[0035]FIG. 5 is a functional block diagram of the IMD of FIG. 4;

[0036]FIG. 6 is a diagram of an IMD illustrating a multiple channelbi-atrial and/or bi-ventricular pacing system coupled with a patient'sheart in accordance with another embodiment of the present invention;

[0037]FIG. 7 is a timing diagram illustrating ventricular safety pacingaccording to the present invention;

[0038]FIG. 8 is a flow diagram for use in describing ventricular pacingfollowing an atrial sensed event according to the present invention;

[0039]FIG. 9 is a flow diagram for use in describing ventricular safetypacing following an atrial paced event according to the presentinvention; and

[0040] FIGS. 10A-10B illustrate examples of the delivery of ventricularsafety pacing stimulus at optimized or programmed SAV and PAV intervalsaccording to the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0041]FIG. 1 is a simplified view of one embodiment of implantablemedical device (“IMD”) 10 of the present invention. IMD 10 shown in FIG.1 is a pacemaker comprising at least one of pacing and sensing leads 16and 18 attached to hermetically sealed enclosure 14 and implanted nearhuman or mammalian heart 8. Pacing and sensing leads 16 and 18, senseelectrical signals attendant to the depolarization and re-polarizationof the heart 8, and further provide pacing pulses for causingdepolarization of cardiac tissue in the vicinity of the distal endsthereof. Leads 16 and 18 may have, for example, unipolar or bipolarelectrodes disposed thereon, as is well known in the art. Examples ofIMD 10 include implantable cardiac pacemakers disclosed in U.S. Pat. No.5,158,078 to Bennett et al., U.S. Pat. No. 5,312,453 to Shelton et al.or U.S. Pat. No. 5,144,949 to Olson.

[0042]FIG. 2 shows connector module 12 and hermetically sealed enclosure14 of IMD 10 located near human or mammalian heart 8. Atrial andventricular pacing leads 16 and 18 extend from connector header module12 to the right atrium and ventricle, respectively, of heart 8. Atrialelectrodes 20 and 21 disposed at the distal end of atrial pacing lead 16are located in the right atrium. Ventricular electrodes 28 and 29 at thedistal end of ventricular pacing lead 18 are located in the rightventricle.

[0043]FIG. 3 is a block diagram illustrating the constituent componentsof IMD 10 in accordance with one embodiment of the present invention,where IMD 10 is pacemaker having a microprocessor-based architecture.IMD 10 is shown as including activity sensor or accelerometer 11, whichis preferably a piezoceramic accelerometer bonded to a hybrid circuitlocated inside enclosure 14. Activity sensor 11 typically (although notnecessarily) provides a sensor output that varies as a function of ameasured parameter relating to a patient's metabolic requirements. Forthe sake of convenience, IMD 10 in FIG. 3 is shown with lead 18 onlyconnected thereto; similar circuitry and connections not explicitlyshown in FIG. 3 apply to lead 16.

[0044] IMD 10 in FIG. 3 is most preferably programmable by means of anexternal programming unit (not shown in the Figures). One suchprogrammer is the commercially available Medtronic Model 9790programmer, which is microprocessor-based and provides a series ofencoded signals to IMD 10, typically through a programming head whichtransmits or telemeters radio-frequency (RF) encoded signals to IMD 10.Such a telemetry system is described in U.S. Pat. No. 5,354,319 toWyborny et al. The programming methodology disclosed in Wyborny et al.'s'319 patent is identified herein for illustrative purposes only. Any ofa number of suitable programming and telemetry methodologies known inthe art may be employed so long as the desired information istransmitted to and from IMD 10.

[0045] As shown in FIG. 3, lead 18 is coupled to node 50 in IMD 10through input capacitor 52. Activity sensor or accelerometer 11 is mostpreferably attached to a hybrid circuit located inside hermeticallysealed enclosure 14 of IMD 10. The output signal provided by activitysensor 11 is coupled to input/output circuit 54. Input/output circuit 54contains analog circuits for interfacing to heart 8, activity sensor 11,antenna 56 and circuits for the application of stimulating pulses toheart 8. The rate of heart 8 is controlled by software-implementedalgorithms stored in microcomputer circuit 58.

[0046] Microcomputer circuit 58 preferably comprises on-board circuit 60and off-board circuit 62. Circuit 58 may correspond to a microcomputercircuit disclosed in U.S. Pat. No. 5,312,453 to Shelton et al. On-boardcircuit 60 preferably includes microprocessor 64, system clock circuit66 and on-board RAM 68 and ROM 70. Off-board circuit 62 preferablycomprises a RAM/ROM unit. On-board circuit 60 and off-board circuit 62are each coupled by data communication bus 72 to digitalcontroller/timer circuit 74. Microcomputer circuit 58 may comprise acustom integrated circuit device augmented by standard RAM/ROMcomponents.

[0047] Electrical components shown in FIG. 3 are powered by anappropriate implantable battery power source 76 in accordance withcommon practice in the art. For the sake of clarity, the coupling ofbattery power to the various components of IMD 10 is not shown in theFigures. Antenna 56 is connected to input/output circuit 54 to permituplink/downlink telemetry through RF transmitter and receiver telemetryunit 78. By way of example, telemetry unit 78 may correspond to thatdisclosed in U.S. Pat. No. 4,556,063 issued to Thompson et al., or tothat disclosed in the above-referenced '319 patent to Wyborny et al. Itis generally preferred that the particular programming and telemetryscheme selected permit the entry and storage of cardiac rate-responseparameters. The specific embodiments of antenna 56, input/output circuit54 and telemetry unit 78 presented herein are shown for illustrativepurposes only, and are not intended to limit the scope of the presentinvention.

[0048] V_(REF) and Bias circuit 82 (see FIG. 3) most preferablygenerates stable voltage reference and bias currents for analog circuitsincluded in input/output circuit 54. Analog-to-digital converter (ADC)and multiplexer unit 84 digitizes analog signals and voltages to provide“real-time” telemetry intracardiac signals and battery end-of-life (EOL)replacement functions. Operating commands for controlling the timing ofIMD 10 are coupled by data bus 72 to digital controller/timer circuit74, where digital timers and counters establish the overall escapeinterval of the IMD 10 as well as various refractory, blanking and othertiming windows for controlling the operation of peripheral componentsdisposed within input/output circuit 54 and for particular employingcertain functionality such as ventricular safety pacing in accordancewith the present invention.

[0049] Digital controller/timer circuit 74 is preferably coupled tosensing circuitry, including sense amplifier 88, peak sense andthreshold measurement unit 90 and comparator/threshold detector 92.Circuit 74 is further preferably coupled to electrogram (EGM) amplifier94 for receiving amplified and processed signals sensed by lead 18.Sense amplifier 88 amplifies sensed electrical cardiac signals andprovides an amplified signal to peak sense and threshold measurementcircuitry 90, which in turn provides an indication of peak sensedvoltages and measured sense amplifier threshold voltages on multipleconductor signal path 67 to digital controller/timer circuit 74. Anamplified sense amplifier signal is then provided tocomparator/threshold detector 92. By way of example, sense amplifier 88may correspond to that disclosed in U.S. Pat. No. 4,379,459 to Stein.

[0050] The electrogram signal provided by EGM amplifier 94 is employedwhen IMD 10 is being interrogated by an external programmer to transmita representation of a cardiac analog electrogram. See, for example, U.S.Pat. No. 4,556,063 to Thompson et al. Output pulse generator 96 providespacing stimuli to patient's heart 8 through coupling capacitor 98, forexample, in response to a pacing trigger signal provided by digitalcontroller/timer circuit 74 each time the escape interval times out, inresponse to an externally transmitted pacing command or in response toother stored commands as is well known in the pacing art. By way ofexample, output amplifier 96 may correspond generally to an outputamplifier disclosed in U.S. Pat. No. 4,476,868 to Thompson.

[0051] The specific embodiments of input amplifier 88, output amplifier96 and EGM amplifier 94 identified herein are presented for illustrativepurposes only, and are not intended to be limiting in respect of thescope of the present invention. The specific embodiments of suchcircuits may not be critical to practicing some embodiments of thepresent invention so long as they provide means for generating astimulating pulse and are capable of providing signals indicative ofnatural or stimulated contractions of heart 8.

[0052] In some embodiments of the present invention, IMD 10 may operatein various non-rate-responsive modes, including, but not limited to,DDD, DDI, VV, VOO and VVT modes. In other embodiments of the presentinvention, IMD 10 may operate in various rate-responsive modes,including, but not limited to, DDDR, DDIR, VVIR, VOOR and VVTR modes.Some embodiments of the present invention are capable of operating inboth non-rate-responsive and rate-responsive modes. Moreover, in variousembodiments of the present invention, IMD 10 may be programmablyconfigured to operate so that it varies the rate at which it deliversstimulating pulses to heart 8 only in response to one or more selectedsensor outputs being generated. Numerous pacemaker features andfunctions not explicitly mentioned herein may be incorporated into IMD10 while remaining within the scope of the present invention.

[0053] The present invention is not limited in scope to dual-sensorpacemakers, and is not limited to IMD's comprising activity or pressuresensors only. Further, the present invention is not limited in scope todual-chamber pacemakers, dual-chamber leads for pacemakers orsingle-sensor or dual-sensor leads for pacemakers. Thus, variousembodiments of the present invention may be practiced in conjunctionwith more than two leads or with multiple-chamber pacemakers, forexample. At least some embodiments of the present invention may beapplied equally well in the contexts of dual-, triple- orquadruple-chamber pacemakers or other types of IMD's. See, for example,U.S. Pat. No. 5,800,465 to Thompson et al.

[0054] IMD 10 may also be a pacemaker-cardioverter-defibrillator (“PCD”)corresponding to any of numerous commercially available implantablePCD's. Various embodiments of the present invention may be practiced inconjunction with PCD's such as those disclosed in U.S. Pat. No.5,545,186 to Olson et al., U.S. Pat. No. 5,354,316 to Keimel, U.S. Pat.No. 5,314,430 to Bardy, U.S. Pat. No. 5,131,388 to Pless and U.S. Pat.No. 4,821,723 to Baker, Jr. et al.

[0055]FIGS. 4 and 5 illustrate one embodiment of IMD 10 and acorresponding lead set of the present invention, where IMD 10 is a PCD.In FIG. 4, the ventricular lead takes the form of leads disclosed inU.S. Pat. Nos. 5,099,838 and 5,314,430 to Bardy, and includes anelongated insulative lead body 1 carrying three concentric coiledconductors separated from one another by tubular insulative sheaths.Located adjacent the distal end of lead 1 are ring electrode 2,extendable helix electrode 3 mounted retractably within insulativeelectrode head 4 and elongated coil electrode 5. Each of the electrodesis coupled to one of the coiled conductors within lead body 1.Electrodes 2 and 3 are employed for cardiac pacing and for sensingventricular depolarizations. At the proximal end of the lead isbifurcated connector 6 which carries three electrical connectors, eachcoupled to one of the coiled conductors. Defibrillation electrode 5 maybe fabricated from platinum, platinum alloy or other materials known tobe usable in implantable defibrillation electrodes and may be about 5 cmin length.

[0056] The atrial/SVC lead shown in FIG. 4 includes elongated insulativelead body 7 carrying three concentric coiled conductors separated fromone another by tubular insulative sheaths corresponding to the structureof the ventricular lead. Located adjacent the J-shaped distal end of thelead are ring electrode 9 and extendable helix electrode 13 mountedretractably within an insulative electrode head 15. Each of theelectrodes is coupled to one of the coiled conductors within lead body7. Electrodes 13 and 9 are employed for atrial pacing and for sensingatrial depolarizations. Elongated coil electrode 19 is provided proximalto electrode 9 and coupled to the third conductor within lead body 7.Electrode 19 preferably is 10 cm in length or greater and is configuredto extend from the SVC toward the tricuspid valve. In one embodiment ofthe present invention, approximately 5 cm of the right atrium/SVCelectrode is located in the right atrium with the remaining 5 cm locatedin the SVC. At the proximal end of the lead is bifurcated connector 17carrying three electrical connectors, each coupled to one of the coiledconductors.

[0057] The coronary sinus lead shown in FIG. 4 assumes the form of acoronary sinus lead disclosed in the above cited '838 patent issued toBardy, and includes elongated insulative lead body 41 carrying onecoiled conductor coupled to an elongated coiled defibrillation electrode21. Electrode 21, illustrated in broken outline in FIG. 4, is locatedwithin the coronary sinus and great vein of the heart. At the proximalend of the lead is connector plug 23 carrying an electrical connectorcoupled to the coiled conductor. The coronary sinus/great vein electrode41 may be about 5 cm in length.

[0058] The implantable PCD is shown in FIG. 4 in combination with leads1, 7 and 41, and lead connector assemblies 23, 17 and 6 inserted intoconnector block 12. Optionally, insulation of the outward facing portionof housing 14 of PCD 10 may be provided using a plastic coating such asparylene or silicone rubber, as is employed in some unipolar cardiacpacemakers. The outward facing portion, however, may be left uninsulatedor some other division between insulated and uninsulated portions may beemployed. The uninsulated portion of housing 14 serves as a subcutaneousdefibrillation electrode to defibrillate either the atria or ventricles.Lead configurations other than those shown in FIG. 4 may be practiced inconjunction with the present invention, such as those shown in U.S. Pat.No. 5,690,686 to Min et al.

[0059]FIG. 5 is a functional schematic diagram of one embodiment of animplantable POD of the present invention. This diagram should be takenas exemplary of the type of device in which various embodiments of thepresent invention may be embodied, and not as limiting, as it isbelieved that the invention may be practiced in a wide variety of deviceimplementations which provided pacing therapies.

[0060] The PCD is provided with an electrode system. If the electrodeconfiguration of FIG. 4 is employed, the electrode configurationcorrespondence may be as follows. Electrode 25 in FIG. 5 includes theuninsulated portion of the housing of the POD. Electrodes 25, 15, 21 and5 are coupled to high voltage output circuit 27, which includes highvoltage switches controlled by CV/defib control logic 29 via control bus31. Switches disposed within circuit 27 determine which electrodes areemployed and which electrodes are coupled to the positive and negativeterminals of the capacitor bank (which includes capacitors 33 and 35)during delivery of defibrillation pulses.

[0061] Electrodes 2 and 3 are located on or in the ventricle and arecoupled to the R-wave amplifier 37, which preferably takes the form ofan automatic gain controlled amplifier providing an adjustable sensingthreshold as a function of the measured R-wave amplitude. A signal isgenerated on R-out line 39 whenever the signal sensed between electrodes2 and 3 exceeds the present sensing threshold.

[0062] Electrodes 9 and 13 are located on or in the atrium and arecoupled to the P-wave amplifier 43, which preferably also takes the formof an automatic gain controlled amplifier providing an adjustablesensing threshold as a function of the measured P-wave amplitude. Asignal is generated on P-out line 45 whenever the signal sensed betweenelectrodes 9 and 13 exceeds the present sensing threshold. The generaloperation of R-wave and P-wave amplifiers 37 and 43 may correspond tothat disclosed in U.S. Pat. No. 5,117,824, to Keimel et al.

[0063] Switch matrix 47 is used to select which of the availableelectrodes are coupled to wide band (0.5-200 Hz) amplifier 49 for use indigital signal analysis. Selection of electrodes is controlled by themicroprocessor 51 via data/address bus 53, which selection may be variedas desired. Signals from the electrodes selected for coupling tobandpass amplifier 49 are provided to multiplexer 55, and thereafterconverted to multi-bit digital signals by A/D converter 57, for storagein random access memory 59 under control of direct memory access circuit61. Microprocessor 51 may employ digital signal analysis techniques tocharacterize the digitized signals stored in random access memory 59 torecognize and classify the patient's heart rhythm employing any of thenumerous signal processing methodologies known in the art.

[0064] The remainder of the circuitry is dedicated to the provision ofcardiac pacing, cardioversion and defibrillation therapies, and, forpurposes of the present invention may correspond to circuitry known tothose skilled in the art. The following exemplary apparatus is disclosedfor accomplishing pacing, cardioversion and defibrillation functions.Pacer timing/control circuitry 63 preferably includes programmabledigital counters which control the basic time intervals associated withDDD, VVI, DVI, VDD, AAI, DDI and other modes of single and dual chamberpacing well known to the art. Circuitry 63 also preferably controlsescape intervals associated with anti-tachyarrhythmia pacing in both theatrium and the ventricle.

[0065] Intervals defined by pacing circuitry 63 include atrial andventricular pacing escape intervals, the refractory periods during whichsensed P-waves and R-waves are ineffective to restart timing of theescape intervals and the pulse widths of the pacing pulses. Thedurations of these intervals are determined by microprocessor 51, inresponse to stored data in memory 59 and are communicated to pacingcircuitry 63 via address/data bus 53. Pacer circuitry 63 also determinesthe amplitude of the cardiac pacing pulses under control ofmicroprocessor 51.

[0066] During pacing, escape interval counters within pacertiming/control circuitry 63 are reset upon sensing of R-waves andP-waves as indicated by signals on lines 39 and 45, and in accordancewith the selected mode of pacing on time-out trigger generation ofpacing pulses by pacer output circuitry 65 and 67, which are coupled toelectrodes 9, 13, 2 and 3. Escape interval counters are also reset ongeneration of pacing pulses and thereby control the basic timing ofcardiac pacing functions, including anti-tachyarrhythmia pacing. Thedurations of the intervals defined by escape interval timers aredetermined by microprocessor 51 via data/address bus 53. The value ofthe count present in the escape interval counters when reset by sensedR-waves and P-waves may be used to measure the durations of R-Rintervals, P-P intervals, P-R intervals and R-P intervals, whichmeasurements are stored in memory 59 and used to detect the presence oftachyarrhythmias.

[0067] Microprocessor 51 most preferably operates as an interrupt drivendevice, and is responsive to interrupts from pacer timing/controlcircuitry 63 corresponding to the occurrence of sensed P-waves andR-waves and corresponding to the generation of cardiac pacing pulses.Those interrupts are provided via data/address bus 53. Any necessarymathematical calculations to be performed by microprocessor 51 and anyupdating of the values or intervals controlled by pacer timing/controlcircuitry 63 take place following such interrupts.

[0068] Detection of atrial or ventricular tachyarrhythmias, as employedin the present invention, may correspond to tachyarrhythmia detectionalgorithms known in the art. For example, the presence of an atrial orventricular tachyarrhythmia may be confirmed by detecting a sustainedseries of short R-R or P-P intervals of an average rate indicative oftachyarrhythmia or an unbroken series of short R-R or P-P intervals. Thesuddenness of onset of the detected high rates, the stability of thehigh rates, and a number of other factors known in the art may also bemeasured at this time. Appropriate ventricular tachyarrhythmia detectionmethodologies measuring such factors are described in U.S. Pat. No.4,726,380 issued to Vollmann et al., U.S. Pat. No. 4,880,005 issued toPless et al. and U.S. Pat. No. 4,830,006 issued to Haluska et al. Anadditional set of tachycardia recognition methodologies is disclosed inthe article “Onset and Stability for Ventricular TachyarrhythmiaDetection in an Implantable Pacer-Cardioverter-Defibrillator” by Olsonet al., published in Computers in Cardiology, Oct. 7-10, 1986, IEEEComputer Society Press, pages 167-170. Atrial fibrillation detectionmethodologies are disclosed in Published PCT Application Ser. No.US92/02829, Publication No. WO92/18198, by Adams et al., and in thearticle “Automatic Tachycardia Recognition”, by Arzbaecher et al.,published in PACE, May-June, 1984, pp. 541-547.

[0069] In the event an atrial or ventricular tachyarrhythmia is detectedand an anti-tachyarrhythmia pacing regimen is desired, appropriatetiming intervals for controlling generation of anti-tachyarrhythmiapacing therapies are loaded from microprocessor 51 into the pacer timingand control circuitry 63, to control the operation of the escapeinterval counters therein and to define refractory periods during whichdetection of R-waves and P-waves is ineffective to restart the escapeinterval counters.

[0070] Alternatively, circuitry for controlling the timing andgeneration of anti-tachycardia pacing pulses as described in U.S. Pat.No. 4,577,633, issued to Berkovits et al., U.S. Pat. No. 4,880,005,issued to Pless et al., U.S. Pat. No. 4,726,380, issued to Vollmann etal. and U.S. Pat. No. 4,587,970, issued to Holley et al., may also beemployed.

[0071] In the event that generation of a cardioversion or defibrillationpulse is required, microprocessor 51 may employ an escape intervalcounter to control timing of such cardioversion and defibrillationpulses, as well as associated refractory periods. In response to thedetection of atrial or ventricular fibrillation or tachyarrhythmiarequiring a cardioversion pulse, microprocessor 51 activatescardioversion/defibrillation control circuitry 29, which initiatescharging of the high voltage capacitors 33 and 35 via charging circuit69, under the control of high voltage charging control line 71. Thevoltage on the high voltage capacitors is monitored via VCAP line 73,which is passed through multiplexer 55 and, in response to reaching apredetermined value set by microprocessor 51, results in generation of alogic signal on Cap Full (CF) line 77 to terminate charging. Thereafter,timing of the delivery of the defibrillation or cardioversion pulse iscontrolled by pacer timing/control circuitry 63. Following delivery ofthe fibrillation or tachycardia therapy, microprocessor 51 returns thedevice to q cardiac pacing mode and awaits the next successive interruptdue to pacing or the occurrence of a sensed atrial or ventriculardepolarization.

[0072] Several embodiments of appropriate systems for the delivery andsynchronization of ventricular cardioversion and defibrillation pulsesand for controlling the timing functions related to them are disclosedin U.S. Pat. No. 5,188,105 to Keimel, U.S. Pat. No. 5,269,298 to Adamset al. and U.S. Pat. No. 4,316,472 to Mirowski et al. However, any knowncardioversion or defibrillation pulse control circuitry is believed tobe usable in conjunction with various embodiments of the presentinvention. For example, circuitry controlling the timing and generationof cardioversion and defibrillation pulses such as that disclosed inU.S. Pat. No. 4,384,585 to Zipes, U.S. Pat. No. 4,949,719 to Pless etal., or U.S. Pat. No. 4,375,817 to Engle et al., may also be employed.

[0073] Continuing to refer to FIG. 5, delivery of cardioversion ordefibrillation pulses is accomplished by output circuit 27 under thecontrol of control circuitry 29 via control bus 31. Output circuit 27determines whether a monophasic or biphasic pulse is delivered, thepolarity of the electrodes and which electrodes are involved in deliveryof the pulse. Output circuit 27 also includes high voltage switcheswhich control whether electrodes are coupled together during delivery ofthe pulse. Alternatively, electrodes intended to be coupled togetherduring the pulse may simply be permanently coupled to one another,either exterior to or interior of the device housing, and polarity maysimilarly be pre-set, as in current implantable defibrillators. Examplesof output circuitry for delivery of biphasic pulse regimens to multipleelectrode systems may be found in U.S. Pat. No. 4,953,551 to Mehra etal. and in U.S. Pat. No. 4,727,877 to Kallock.

[0074] An example of circuitry which may be used to control delivery ofmonophasic pulses is disclosed in U.S. Pat. No. 5,163,427 to Keimel.Output control circuitry similar to that disclosed in the above-citedpatent issued to Mehra et al. or U.S. Pat. No. 4,800,883 to Winstrom,may also be used in conjunction with various embodiments of the presentinvention to deliver biphasic pulses.

[0075]FIG. 6 is a schematic representation of an implantable medicaldevice (IMD) 114 that includes a four-channel cardiac pacemaker such asthat described in U.S. Pat. No. 6,070,101 to Struble et al. entitled“Multiple Channel, Sequential, Cardiac Pacing Systems,” issued May 30,2000. The inline connector 113 of a right atrial lead 116 is fitted intoa bipolar bore of IMD connector block 112 and is coupled to a pair ofelectrically insulated conductors within lead body 115 that areconnected with distal tip right atrial pace-sense electrode 119 andproximal ring right atrial pace-sense electrode 121. The distal end ofthe right atrial lead 116 is attached to the right atrial wall by aconventional attachment mechanism 117. Bipolar endocardial rightventricle lead 132 is passed through the vein into the right atrialchamber of the heart 8 and into the right ventricle where its distalring and tip right ventricular pace-sense electrodes 138 and 140 arefixed in place in the apex by a conventional and distal attachmentmechanism 141. The right ventricular lead 132 is formed with an inlineconnector 134 fitting into a bipolar bore of IMD connector block 112that is coupled to a pair of electrically insulated conductors withinlead body 136 and then connected with distal tip right ventricularpace-sense electrode 140 and proximal ring right ventricular pace-senseelectrode 138.

[0076] In this particular illustrative embodiment, although other typesof leads may be used, a quadripolar, endocardial left ventricularcoronary sinus (CS) lead 152 is passed through a vein into the rightatrial chamber of the heart 8, into the CS, and then inferiorly in thegreat vein to extend to the distal pair of left ventricular CSpace-sense electrodes 148 and 150 alongside the left ventricular chamberand leave the proximal pair of left atrial CS pace-sense electrodes 128and 130 adjacent the left atrial chamber. The left ventricular CS lead152 is formed with a four-conductor lead body 156 coupled at theproximal end to a bifurcated inline connector 154 fitting into a pair ofbipolar bores of IMD connector block 112. The four electricallyinsulated lead conductors in left ventricular CS lead body 156 areseparately connected with one of the distal pair of left ventricular CSpace-sense electrodes 148 and 150 and the proximal pair of left atrialCS pace-sense electrodes 128 and 130.

[0077] The IMD 114 may comprise, for example, similar circuitry andconnections as shown in FIG. 3 for each of the multiple leads toestablish the multiple pacing/sensing channels provided for eachrespective pair of pace-sense electrodes associated with each chamber ofthe heart as shown in FIG. 6. For the sake of convenience, suchcircuitry is not described further. For example, channel circuitry forpacing/sensing the left atrial chamber is associated with pace-senseelectrodes 28 and 30 adjacent the left atrium. One skilled in the artwill recognize that each sensing/pacing channel may include a senseamplifier and pace output pulse generator coupled through the respectivepacing/sensing lead. Although the pacing system shown in FIG. 6, shallnot be described in detail for simplicity purposes, it will berecognized that multiple chambers may be paced/sensed via respectivechannels for such chambers. As such, for example, bi-atrial and/orbi-ventricular pacing may be performed as would be readily apparent toone skilled in the art.

[0078] With various embodiments of medical devices, e.g., implantablemedical devices, described above, it will become apparent from thedescription below that the present invention may be applied to anyventricular pacing system, e.g., dual chamber pacing system. Forexample, the present invention may be applied to a three-chamberatrial-bi-ventricular pacing apparatus, a dual chamber pacing apparatus,a dual chamber defibrillator, etc. In other words, the present inventionmay be applied to any implantable medical device that provides pacing ofat least one ventricle, whether such pacing is based on paced atrialevents or sensed atrial events. For example, some devices that may bemodified to include the ventricular safety pacing techniques accordingto the present invention may include, for example, the InSync,InSync-ICD, or InSync III three chamber atrial-bi-ventricular pacers;all VDD(R)/DDD(R) pacemakers including dual chamber right atrial/leftventricular pacers; Jewel DR DDD(R)-ICD; dual chamber (right atrial/leftventricular) defibrillators, and three chamber DDD(R)-ICD pacing devicesavailable from Medtronic Inc.

[0079] Generally, FIG. 7 shows a timing diagram 200 that is illustrativeof the ventricular safety pacing (VSP) features according to the presentinvention. Such features shall be described with respect to a pacing andsensing system operating in DDD mode. However, one skilled in the artwill readily recognize that other modes and systems may utilize thepresent invention as described herein.

[0080] Delivery of ventricular pacing, e.g., bi-ventricular pacing, atoptimized SAV and PAV delays is ensured according to the presentinvention without such pacing being inhibited by false ventricularsensing, e.g., sensing of cross-talk, post atrial pacing ringing, etc.Such ensured delivery of ventricular pacing is accomplished through useof a VSP window 204 as shown in timing diagram 200 of FIG. 7.

[0081] As shown in FIG. 7, following an atrial sensed or paced event201, a ventricular blanking period 210 occurs. This blanking period is avery brief interval initiated by the atrial paced/sensed event 201during which ventricular sensing cannot occur. Typically, an atrialstimulus disables ventricular sensing for a 25-35 millisecond period toprevent inadvertent sensing of the atrial stimulus by a ventricularchannel, thereby preventing inappropriate inhibition of delivery ofventricular stimulus. If a blanking period 210 was not used, cross-talkbetween the atrial and ventricular channels might lead to theundesirable inhibition of the delivery of ventricular stimulus by thepacemaker.

[0082] The blanking period 210 falls within the AV interval, e.g., pacedAV (PAV) interval 202 and/or sensed AV (SAV) interval 203. The AVinterval is the interval between either an atrial sensed event or anatrial paced event 201 and delivery of a ventricular stimulus. Forexample, following an atrial paced event 201 is a PAV interval 202extending from the atrial paced event 201 to delivery of a ventricularstimulus 209. Likewise, the SAV interval 203 is the interval between asensed atrial event 201 and delivery of ventricular stimulus 207.

[0083] In general, the PAV interval 202 is a programmed time delaybetween the paced atrial event 201 and when delivery of ventricularstimulus 209 is to occur thereafter (if no intrinsic ventricularactivity is sensed). Likewise, the SAV interval 203 is a programmed AVdelay between a sensed atrial event 201 and when ventricular stimulus isdelivered 207 thereafter (if no intrinsic ventricular activity issensed). As such, as used hereinafter, the SAV delay and the PAV delayrepresent the programmed AV interval initiated by an atrial sensed oratrial paced event 201, respectively, and such terms “AV interval” and“AV delay” are used interchangeable herein.

[0084] A sensed atrial event or beat, i.e., a P-wave, will start aprogrammed SAV delay. For example, if such depolarization of the atriumis sensed, then a SAV timer is started. If the atrium is paced, such anatrial beat will start a programmed PAV delay. For example, a programmedPAV interval, e.g., a software interval, may be initiated. One willrecognize that various hardware and software may be used to implementthe various timing intervals of the present invention. For example, thecircuitry shown in FIG. 3 may be used to implement the presentinvention, e.g., the processing circuitry and timing circuitry.

[0085] When VSP according to the present invention is turned on,programmed on, or, in other words, when this particular VSP feature isenabled, then a VSP window 204 is defined during at least an initialportion of a sensed AV delay 203 or a paced AV delay 202. The VSP window204 is the time period, e.g., 0 to 110 milliseconds, of the AV interval,e.g., PAV delay or SAV delay, during which if sensed ventricular eventsor activity are detected, delivery of ventricular stimulus is committedupon the expiration of the PAV or SAV delay. In other words, the VSPwindow 204 is defined such that upon ventricular sensing during the VSPwindow 204, VSP pacing at the termination of the SAV delay or PAV delayoccurs irrespective of any other signals sensed during the SAV delay orPAV delay.

[0086] Upon occurrence of an atrial paced event 201, a PAV delay 202 isinitiated. VSP window 204 is defined during an initial portion of thePAV delay 202 with a remainder portion 213 subsequent to the initialportion occupied by VSP window 203. Likewise, upon occurrence of asensed atrial event 201, VSP window 204 is defined during an initialportion of SAV delay 203 with a remainder portion 211 being subsequentto the initial portion occupied by VSP window 204.

[0087] Therefore, in other words, for example, the SAV programmed delayor interval 203 can be looked at as including an initial time period,e.g., 0-110 milliseconds, corresponding to the VSP window 204 and aremainder time period 211 before delivery of the ventricular stimulus207 (assuming no intrinsic events are sensed in the remainder timeperiod 211). For example, if the SAV delay 203 is programmed to 120milliseconds, then the first 110 milliseconds may be the VSP window 204,and an additional remainder 10 milliseconds will be the remainderportion 211 that follows the VSP window 204 prior to termination of theSAV programmed delay 203.

[0088] Likewise, the PAV programmed delay or interval 202 can be lookedat as including an initial time period, e.g., 0-110 milliseconds,corresponding to the VSP window 204 and a remainder time period 213before delivery of the ventricular stimulus 209 (assuming no intrinsicevents are sensed in the remainder time period 213). For example, if thePAV delay 202 is programmed to 150 milliseconds, then the first 110milliseconds may be the VSP window 204, and an additional 40milliseconds will be the remainder portion 213 that follows the VSPwindow 204 prior to termination of the PAV programmed delay 202.

[0089] Preferably, VSP window 204 is 110 milliseconds. Further,preferably, the SAV delay 203 is greater in length than the VSP window204. Likewise, preferably, the PAV delay 202 is greater in length thanthe VSP window 204.

[0090] Generally, the PAV delay 202 as optimized and programmed issignificantly longer than the SAV delay 203. For example, an anticipatedrange in most patients for optimized SAV/PAV delays is about SAV 80-130milliseconds/PAV 130-180 milliseconds. For example, the differencebetween SAV delay and PAV delay is typically 30 milliseconds to 50milliseconds. Further, for example, if the SAV delay is equal to 120milliseconds, then the PAV delay is typically equal to about 150-170milliseconds. The PAV delay is usually longer than SAV delay becausethere is a time period between delivery of pacing stimulus to the atriumand the onset (i.e., lag time) of the atrial depolarization (i.e.,P-wave) which is absent when an atrial event is sensed. In sensed atrialevents, the stimulus has already been delivered, and only the resultingP-wave depolarization signal is seen. This lag time is typically about30-50 milliseconds. A lag time of about 50 milliseconds for SAV delayrelative to PAV delay is generally necessary to accommodate for slowerconduction in HF patients with conduction delays.

[0091] As shown in FIG. 7, preferably the VSP window 204 is defined atthe same fixed interval for both a PAV delay 202 and an SAV delay 203.Further, as indicated above, the VSP window is preferably 110milliseconds as, generally, any signal sensed on the ventricular channelwithin 110 milliseconds of any atrial event, cannot be a truly conductedintrinsic ventricular beat because such an intrinsic ventricular eventwould take significantly longer to occur. Therefore, this sensed signalduring such a 110 millisecond interval must be a false signal. Such asignal sensed during this time period may be, for example, post atrialringing, EMI, PVC, etc. As a pacemaker cannot discriminate whether it isa false or intrinsic ventricular signal, ventricular inhibition based onan early (0 to 110 millisecond) signal from any uncertain source must beprevented at all times.

[0092] Therefore, according to the present invention, if any ventricularsensing occurs at all in the VSP window 204, then a ventricular stimulusis always committed at the termination of the SAV or PAV delay,irrespective of any further sensing which might occur at a later time,e.g., in the remainder of the SAV or PAV. In other words, if a falsesignal, e.g., EMI, is sensed in the VSP window 204, then this samesignal may be sensed in a later period. Such a later false sensingsignal cannot be allowed to lead to ventricular pacing inhibition. Withuse of committed pacing following a sensed event in the VSP window 204as described above, inhibition based on such a false signal isprevented.

[0093] The optimized or programmed PAV and SAV delays are set at lengthsdetermined specifically on a patient-by-patient basis. Such values areset to provide optimized pacing delivery for such patients.

[0094]FIGS. 8 and 9 shall be used to describe VSP according to thepresent invention. FIG. 8 shows a VSP method 220 following theoccurrence of an atrial sensed event (block 222), whereas FIG. 9 shows aVSP method 250 following the occurrence of an atrial paced event (block252). An AV delay may be initiated by occurrence of either an atrialsensed event (block 222, see FIG. 8) or an atrial paced event (block252, see FIG. 9). The AV delay is a programmed time period whose lengthis varied depending upon whether the AV delay is initiated by theoccurrence of the atrial sensed event (block 222) or is initiated by theoccurrence of an atrial paced event (block 252). Although the presentmethod is described relative to VSP following both the occurrence ofatrial sensed events and atrial paced events, it will be recognized thatthe present invention may also be limited to one or the other. In otherwords, a VSP feature may be turned on, programmed on, or enabled forjust paced sensed events or VSP may be turned on, programmed on, orenabled for just atrial sensed events. However, preferably, according tothe present invention, VSP is enabled for and occurs following bothatrial sensed events and atrial paced events.

[0095] As shown in FIG. 8, upon the occurrence of an atrial sensed event(block 222), an SAV delay 203 is initiated (block 224). In addition, asshown in block 226, a VSP window 204 is defined during the initialportion of the SAV delay 203. During the SAV delay 203, sensingcircuitry provides for the sensing of ventricular events (block 228).For example, ventricular activity or ventricular sensed events 206(which may be false sense events resulting from cross talk, leaddislodgement, etc.) may be sensed during VSP window 204.

[0096] As the ventricular chambers are being monitored for ventricularevents, if ventricular events are detected during VSP window 204 (block230), such detected ventricular events initiate committed VSP (block232). Once committed VSP is initiated (block 232), then upon expirationof SAV delay (block 238), a VSP ventricular pace is delivered (block240).

[0097] If no ventricular events are sensed during the VSP window (block230), the ventricular chambers are still being monitored for intrinsicactivity. As such, ventricular events may be sensed during the remainderportion of the SAV interval (block 234).

[0098] If a ventricular event is sensed during the remainder portion ofthe SAV interval (block 234) and no ventricular events were detectedduring the VSP window 204, delivery of ventricular stimulus at thetermination of the SAV delay 203 is inhibited (block 236). In such acircumstance, pacing is unnecessary as proper depolarization of theventricle has occurred intrinsically. In other words, an intrinsicventricular event controls heart activity. Such intrinsic activitycontrolling the heart is desirable.

[0099] If, however, a sensed ventricular event is not detected duringthe remainder (i.e., time portion 211) of the SAV interval (block 234)and no ventricular events were detected during the VSP window 204, thenupon expiration of the SAV delay (block 238), ventricular stimulus isdelivered (block 240).

[0100] In summary, and with reference to FIG. 7, ventricular stimulus isdelivered 207 upon expiration of the SAV delay 203 if no ventricularevents are sensed during the VSP window 204 defined during the initialportion of the SAV delay 203 and the remainder portion 211 thereof. Ifventricular activity or events 206 are sensed during the VSP window,these events commit the pacemaker to the delivery of (and do not inhibitdelivery of) ventricular stimulus upon expiration of the SAV delay 203.In such a manner, committed ventricular pacing therapy is provided atoptimized and programmed SAV delay 203. However, if a ventricular eventis sensed during the remainder portion 211 of the SAV interval 203 andno ventricular events were detected during the VSP window 204, deliveryof ventricular stimulus at the termination of the SAV delay 203 isinhibited, e.g., intrinsic activity controls.

[0101] As shown in FIG. 9, upon the occurrence of an atrial paced event(block 252), a PAV delay 202 is initiated (block 254). In addition, asshown in block 256, a VSP window 204 is defined during the initialportion of the PAV delay 202. During the PAV delay 202, sensingcircuitry provides for the sensing of ventricular events (block 258).For example, ventricular activity or ventricular sensed events 206(which may be false sense events resulting from cross talk, leaddislodgement, etc.) may be sensed during VSP window 204.

[0102] As the ventricular chambers are being monitored for ventricularevents, if ventricular events are detected during VSP window 204 (block260), such detected ventricular events initiate committed VSP (block262). Once committed VSP is initiated (block 262), then upon expirationof PAV delay (block 268), a VSP ventricular pace is delivered (block270).

[0103] If no ventricular events are sensed during the VSP window (block260), the ventricular chambers are still being monitored for intrinsicactivity. As such, ventricular events may be sensed during the remainderportion (i.e., time portion 213) of the PAV interval (block 264).

[0104] If a ventricular event is sensed during the remainder portion(i.e., time portion 213) of the PAV interval (block 264) and noventricular events were detected during the VSP window 204, delivery ofventricular stimulus at the termination of the PAV delay 202 isinhibited (block 266). In such a circumstance, pacing is unnecessary asproper depolarization of the ventricle has occurred intrinsically. Inother words, an intrinsic ventricular event controls heart activity.Such intrinsic activity controlling the heart is desirable.

[0105] If, however, a sensed ventricular event is not detected duringthe remainder of the PAV interval (block 264) and no ventricular eventswere detected during the VSP window 204, then upon expiration of the PAVdelay (block 268), ventricular stimulus is delivered (block 270).

[0106] In summary, and with reference to FIG. 7, ventricular stimulus isdelivered 209 upon expiration of the PAV delay 202 if no ventricularevents are sensed during the VSP window 204 defined during the initialportion of the PAV delay 202 and the remainder portion 213 thereof. Ifventricular activity or events 206 are sensed during the VSP window,these events commit the pacemaker to the delivery of (and do not inhibitdelivery of) ventricular stimulus upon expiration of the PAV delay 202.In such a manner, committed ventricular pacing therapy is provided atoptimized and programmed PAV delay 202. However, if a ventricular eventis sensed during the remainder portion 213 of the PAV interval 202 andno ventricular events were detected during the VSP window 204, deliveryof ventricular stimulus at the termination of the PAV delay 202 isinhibited, e.g., intrinsic activity controls.

[0107]FIGS. 10A and 10B provide two illustrative examples of VSP for anSAV interval (as shown in FIG. 10A) and for a PAV interval (as shown inFIG. 10B). FIG. 10A shows a SAV delay 302 of 120 milliseconds initiatedupon the occurrence of an atrial sensed event 306. Also upon occurrenceof the atrial sensed event 306, VSP window 304 of 110 milliseconds isdefined with a remainder period 314 subsequent thereto in the SAV delay302. A ventricular event 308 is sensed during the VSP window 304. Assuch, this sensed ventricular event 308 commits the pacing apparatus todeliver a VSP pulse 310 at the termination 312 of the SAV interval ordelay 302.

[0108] Likewise, FIG. 10B, shows a PAV delay 354 of 150 millisecondsinitiated upon the occurrence of an atrial paced event 352. Also uponoccurrence of the atrial paced event 352, VSP window 356 of 110milliseconds is defined with a remainder period 364 subsequent theretoin the PAV delay 354. A ventricular event 358 is sensed during the VSPwindow 356. As such, this sensed ventricular event 358 commits thepacing apparatus to deliver a VSP pulse 360 at the termination 362 ofthe PAV interval or delay 354.

[0109] All patents and references cited herein are incorporated in theirentirety as if each were incorporated separately. This invention hasbeen described with reference to illustrative embodiments and is notmeant to be construed in a limiting sense. As described previously, oneskilled in the art will recognize that various other illustrativeapplications including, but not limited to, bi-ventricular pacing, mayutilize the VSP techniques described herein. Various modifications ofthe illustrative embodiments, as well as additional embodiments of theinvention, will be apparent to persons skilled in the art upon referenceto this description.

What is claimed is:
 1. A method of pacing for use in a medical device,the method comprising: providing a sensed AV delay following an atrialsensed event, wherein the sensed AV delay is a predetermined time periodinitiated by the atrial sensed event; defining a ventricular safetypacing window during at least an initial portion of the sensed AV delay,wherein the sensed AV delay further comprises a remainder portionthereof subsequent to the initial portion; sensing ventricular eventsduring the sensed AV delay; delivering ventricular stimulus uponexpiration of the sensed AV delay if no ventricular events are sensedduring the ventricular safety pacing window defined during the initialportion of the sensed AV delay and the remainder portion thereof;inhibiting the delivery of ventricular stimulus upon expiration of thesensed AV delay if no ventricular events are sensed during theventricular safety pacing window but a ventricular event is sensedduring the remainder portion of the sensed AV delay; and committing tothe delivery of a ventricular stimulus upon expiration of the sensed AVdelay if a ventricular event is sensed during the ventricular safetypacing window.
 2. The method of claim 1, wherein the sensed AV delay isequal to or greater than the ventricular safety pacing window.
 3. Themethod of claim 1, wherein the ventricular safety pacing window duringthe initial portion of the sensed AV delay is 110 milliseconds.
 4. Themethod of claim 1, wherein the method further comprises: providing apaced AV delay following an atrial paced event, wherein the paced AVdelay is a predetermined time period initiated by the atrial pacedevent; defining a ventricular safety pacing window during at least aninitial portion of the paced AV delay, wherein the paced AV delayfurther comprises a remainder portion thereof subsequent to the initialportion; sensing ventricular events during the paced AV delay;delivering ventricular stimulus upon expiration of the paced AV delay ifno ventricular events are sensed during the ventricular safety pacingwindow defined during the initial portion of the paced AV delay and theremainder portion thereof; inhibiting the delivery of ventricularstimulus upon expiration of the paced AV delay if no ventricular eventsare sensed during the ventricular safety pacing window but a ventricularevent is sensed during the remainder portion of the paced AV delay; andcommitting to the delivery of a ventricular stimulus upon expiration ofthe paced AV delay if a ventricular event is sensed during theventricular safety pacing window.
 5. The method of claim 4, wherein theventricular safety pacing window during the initial portion of the pacedAV delay is 110 milliseconds.
 6. The method of claim 4, wherein thepaced AV delay is equal to or greater than the ventricular safety pacingwindow.
 7. The method of claim 4, wherein the ventricular safety pacingwindow during the initial portion of the paced AV delay is the same asthe ventricular safety pacing window during the initial portion of thesensed AV delay.
 8. The method of claim 4, wherein the paced AV delayand the sensed AV delay are both greater than the ventricular safetypacing window.
 9. The method of claim 8, wherein the paced AV delay isgreater than the sensed AV delay.
 10. The method of claim 4, wherein theventricular safety pacing window during the initial portion of the pacedAV delay and the ventricular safety pacing window during the initialportion of the sensed AV delay are both 110 milliseconds.
 11. The methodof claim 1, wherein delivering ventricular stimulus comprises deliveringbi-ventricular stimulus.
 12. The method of claim 1, wherein the medicaldevice is an implantable medical device.
 13. The method of claim 12,wherein the implantable medical device includes an implantable medicaldevice selected from one of a pacemaker, a defibrillator, apacemaker/cardioverter/defibrillator, and a cardioverter/defibrillator.14. A method of pacing for use in a medical device, the methodcomprising: providing an AV delay initiated by occurance of either anatrial sensed event or an atrial paced event, wherein the AV delay is aprogrammed time period whose length is varied depending upon whether theAV delay is intiated by the occurance of an atrial sensed event or isinitiated by the occurance of an atrial paced event; defining aventricular safety pacing window during at least an initial portion ofthe AV delay; sensing ventricular events during the AV delay; deliveringventricular stimulus upon expiration of the AV delay if no ventricularevents are sensed during the AV delay; and committing to the delivery ofventricular stimulus upon expiration of the AV delay if a ventricularevent is sensed during the ventricular safety pacing window.
 15. Themethod of claim 14, wherein the method further comprises inhibitingdelivery of ventricular stimulus upon expiration of the AV delay if aventricular event is not sensed during the ventricular safety pacingwindow but a ventricular event is sensed during a subsequent portion ofthe AV interval.
 16. The method of claim 14, wherein providing the AVdelay includes providing a sensed AV delay initiated by occurance of anatrial sensed event or providing a paced AV delay initiated by occuranceof an atrial paced event.
 17. The method of claim 16, wherein theventricular safety pacing window during the initial portion of the pacedAV delay and the ventricular safety pacing window during the initialportion of the sensed AV delay are 110 milliseconds.
 18. The method ofclaim 16, wherein the paced AV delay and the sensed AV delay are bothgreater than the ventricular safety pacing window.
 19. The method ofclaim 16, wherein the paced AV delay is greater than the sensed AVdelay.
 20. The method of claim claim 14, wherein delivering ventricularstimulus comprises delivering bi-ventricular stimulus.
 21. The method ofclaim 14, wherein the medical device is an implantable medical device.22. The method of claim 14, wherein the implantable medical devicecomprises an implantable medical device selected from one of apacemaker, a defibrillator, a pacemaker/cardioverter/defibrillator, anda cardioverter/defibrillator.
 23. A method of pacing in a dual chamberpacing apparatus, the method comprising: providing a sensed AV delayfollowing an atrial sensed event, wherein the sensed AV delay is apredetermined time period initiated by the atrial sensed event; defininga ventricular safety pacing window during at least an initial portion ofthe sensed AV delay; and committing to the delivery of a ventricularstimulus upon expiration of the sensed AV delay if a ventricular eventis sensed during the ventricular safety pacing window.
 24. The method ofclaim 23, wherein the method further comprises: delivering ventricularstimulus upon expiration of the sensed AV delay if no ventricular eventsare sensed during the sensed AV delay; and inhibiting the delivery ofventricular stimulus upon expiration of the sensed AV delay if noventricular events are sensed during the ventricular safety pacingwindow but a ventricular event is sensed during a subsequent portion ofthe sensed AV delay.
 25. The method of claim 23, wherein the ventricularsafety pacing window during the initial portion of the sensed AV delayis 110 milliseconds.
 26. The method of claim 23, wherein the sensed AVdelay is equal to or greater than the ventricular safety pacing window.27. The method of claim 23, wherein the method further comprises:providing a paced AV delay following an atrial paced event, wherein thepaced AV delay is a predetermined time period initiated by the atrialpaced event; defining a ventricular safety pacing window during at leastan initial portion of the paced AV delay; and committing to the deliveryof a ventricular stimulus upon expiration of the paced AV delay if aventricular event is sensed during the ventricular safety pacing windowthereof.
 28. The method of claim 27, wherein the ventricular safetypacing window during the initial portion of the paced AV delay is 110milliseconds.
 29. The method of claim 27, wherein the paced AV delay isgreater than the ventricular safety pacing window.
 30. The method ofclaim 27, wherein the paced AV delay and the sensed AV delay are bothgreater than the ventricular safety pacing window.
 31. The method ofclaim 30, wherein the paced AV delay is greater than the sensed AVdelay.
 32. The method of claim 26, wherein the ventricular safety pacingwindow during the initial portion of the paced AV delay and theventricular safety pacing window during the initial portion of thesensed AV delay are both 110 milliseconds.
 33. The method of claim 23,wherein delivering ventricular stimulus comprises deliveringbi-ventricular stimulus.
 34. The method of claim 23, wherein the dualchamber pacing apparatus comprises an implantable medical deviceselected from one of a pacemaker, a defibrillator, apacemaker/cardioverter/defibrillator, and a cardioverter/defibrillator.35. A dual chamber pacing apparatus, the apparatus comprising: atrialpacing and sensing circuitry to generate atrial pacing pulses and senseatrial events; ventricular pacing and sensing circuitry to generateventricular pacing pulses and sense ventricular events; and controlcircuitry operable to: provide a sensed AV delay following an atrialsensed event detected using the atrial sensing circuitry, wherein thesensed AV delay is a predetermined time period initiated by the atrialsensed event; define a ventricular safety pacing window during at leastan initial portion of the sensed AV delay, wherein the sensed AV delayfurther comprises a remainder portion thereof subsequent to the initialportion; detect ventricular events using the ventricular sensingcircuitry during the sensed AV delay; control delivery of ventricularstimulus using the ventricular pacing circuitry upon expiration of thesensed AV delay such that: if no ventricular events are detected duringthe ventricular safety pacing window defined during the initial portionof the sensed AV delay and the remainder portion thereof thenventricular stimulus is delivered at the expiration of the sensed AVdelay; if a ventricular event is not detected during the ventricularsafety pacing window but a ventricular event is detected during theremainder portion of the sensed AV delay then delivery of ventricularstimulus upon expiration of the sensed AV delay is inhibited; and if aventricular event is sensed by the ventricular sensing circuitry duringthe ventricular safety pacing window then the ventricular pacingcircuitry is committed to delivery of ventricular stimulus uponexpiration of the sensed AV delay.
 36. The apparatus of claim 35,wherein the ventricular safety pacing window during the initial portionof the sensed AV delay is 110 milliseconds.
 37. The apparatus of claim35, wherein the sensed AV delay is equal to or greater than theventricular safety pacing window.
 38. The apparatus of claim 35, whereinthe control circuitry is further operable to: provide a paced AV delayfollowing an atrial paced event, wherein the paced AV delay is apredetermined time period initiated by the occurance of an atrial pacedevent associated with the generation of atrial pacing pulses by theatrial pacing circuitry; define a ventricular safety pacing windowduring at least an initial portion of the paced AV delay, wherein thepaced AV delay further comprises a remainder portion thereof subsequentto the initial portion; detect ventricular events during the paced AVdelay; control delivery of ventricular stimulus using the ventricularpacing circuitry upon expiration of the paced AV delay such that: if noventricular events are detected during the ventricular safety pacingwindow defined during the initial portion of the paced AV delay and theremainder portion thereof then ventricular stimulus is delivered at theexpiration of the paced AV delay; if a ventricular event is not detectedduring the ventricular safety pacing window but a ventricular event isdetected during the remainder portion of the paced AV delay thendelivery of ventricular stimulus upon expiration of the paced AV delayis inhibited; and if a ventricular event is sensed by the ventricularsensing circuitry during the ventricular safety pacing window then theventricular pacing circuitry is committed to delivery of ventricularstimulus upon expiration of the paced AV delay.
 39. The apparatus ofclaim 38, wherein the ventricular safety pacing window during theinitial portion of the paced AV delay is 110 milliseconds.
 40. Theapparatus of claim 38, wherein the paced AV delay is greater than theventricular safety pacing window.
 41. The apparatus of claim 38, whereinthe ventricular safety pacing window during the initial portion of thepaced AV delay is the same as the ventricular safety pacing windowduring the initial portion of the sensed AV delay.
 42. The apparatus ofclaim 38, wherein the paced AV delay and the sensed AV delay are bothgreater than the ventricular safety pacing window.
 43. The apparatus ofclaim 42, wherein the paced AV delay is greater than the sensed AVdelay.
 44. The apparatus of claim 38, wherein the ventricular safetypacing window during the initial portion of the paced AV delay and theventricular safety pacing window during the initial portion of thesensed AV delay are both 110 milliseconds.
 45. The apparatus of claim38, wherein ventricular pacing and sensing circuitry comprisesbi-ventricular pacing and sensing circuitry to generate bi-ventricularpacing pulses and sense ventricular events.
 46. The apparatus of claim35, wherein the dual chamber pacing apparatus comprises an implantablemedical device selected from one of a pacemaker, a defibrillator, apacemaker/cardioverter/defibrillator, and a cardioverter/defibrillator.47. A dual chamber pacing apparatus, the apparatus comprising: atrialpacing and sensing circuitry to generate atrial pacing pulses and senseatrial events; ventricular pacing and sensing circuitry to generateventricular pacing pulses and sense ventricular events; and controlcircuitry operable to: provide an AV delay initiated by occurance ofeither an atrial sensed event detected by the atrial sensing circuitryor an atrial paced event associated with the generation of atrial pacingpulses using the atrial pacing circuitry, wherein the AV delay is aprogrammed time period whose length is varied depending upon whether theAV delay is intiated by the occurance of an atrial sensed event or isinitiated by the occurance of an atrial paced event; define aventricular safety pacing window during at least an initial portion ofthe AV delay; detect ventricular events during the AV delay; and committo delivery of ventricular stimulus by the ventricular pacing circuitryupon expiration of the AV delay if a ventricular event is sensed by theventricular sensing circuitry during the ventricular safety pacingwindow.
 48. The apparatus of claim 47, wherein the control circuitry isfurther operable to: initiate delivery of ventricular stimulus using theventricular pacing circuitry upon expiration of the AV delay if noventricular events are sensed during the AV delay; and inhibit thedelivery of ventricular stimulus upon expiration of the AV delay if aventricular event is not sensed during the ventricular safety pacingwindow but a ventricular event is sensed during a subsequent portion ofthe AV delay.
 49. The apparatus of claim 47, wherein the controlcircuitry is further operable to provide a sensed AV delay initiated byoccurance of an atrial sensed event or provide a paced AV delayinitiated by occurance of an atrial paced event.
 50. The apparatus ofclaim 49, wherein the ventricular safety pacing window during theinitial portion of the paced AV delay and the ventricular safety pacingwindow during the initial portion of the sensed AV delay are both 110milliseconds.
 51. The apparatus of claim 49, wherein the paced AV delayand the sensed AV delay are both greater than the ventricular safetypacing window.
 52. The apparatus of claim 49, wherein the paced AV delayis greater than the sensed AV delay.
 53. The apparatus of claim 47,wherein ventricular pacing and sensing circuitry comprisesbi-ventricular pacing and sensing circuitry to generate bi-ventricularpacing pulses and sense ventricular events.
 54. The apparatus of claim47, wherein the dual chamber pacing apparatus comprises an implantablemedical device selected from one of a pacemaker, a defibrillator, apacemaker/cardioverter/defibrillator, and a cardioverter/defibrillator.